To add additional photos to your listing from the app:
Click on "My Sold Books" in the Account page
Select the sale
Click the button labeled "Edit" in the bottom right corner
Click "Add Photos"
You can take new photos with the camera that opens, or add saved photos from your phone by clicking the landscape in the bottom left corner. Each listing can have up to 10 photos.
Remember to click "Save Changes" at the bottom of the screen when you're done!